Electric screwdriver

ABSTRACT

A universal motor which operates an electric tool is controlled by a phase-gating controller for determining variable rotational speeds. The phase-gating controller can be switched over to wave-packet-controlled phase-gating control mode in order to be able to switch over between a continuous speed-torque characteristic and pulsating operation.

This application is a National Stage of International ApplicationPCT/EP95/03 923 under 35 USC § 371, filed Oct.5, 1995.

BACKGROUND OF INVENTION

The present invention relates generally to the field of electricscrewdrivers and, more particularly, to an electric screwdriver, ortool, which is operated by an AC universal motor and which incorporatesa phase-gating controller.

In phase-gating controllers, according to the prior art, each half waveof the AC voltage is used either as a full wave or as a phase-gatedwave, in order to achieve operation which is as uniform as possible andhas a continuous speed characteristic.

Known electric screwdrivers are equipped with this type of controller.The use of continuous rotational speeds when screws, used in theconstruction of buildings or the like, are being driven home lead toejection forces on the screwdriver which have to be counteractedmanually. It is extremely difficult to drive building screws home flushin a continuous fashion. The incorporation of a pre-selected value oftorque to overcome this problem is not practical for fasteners in woodmaterials or on wood structures, since virtually every screwingoperation requires a different torque.

It is also known to provide an electric motor operated on an AC systemwith a squirrel-cage rotor for driving a circular saw with a blankingcontroller which connects the AC voltage intermittently to the motor asshown in German Patent DE 41 30 532A1. In this known circuitarrangement, the aim is to reduce the power consumption under specificoperating condutions, such as during idling, in order to maintain therotational speed corresponding to full operation. On the other hand, itis also possible to provide a reduction in rotational speed and torquein order to reduce the power consumption in partial-load operation. Sucha blanking controller does not come into consideration from the point ofview of power reduction for electric tools used in manual operationbecause the centrifugal masses are too small for continuous operation.Furthermore, a squirrel-cage induction motor cannot output a usefultorque up to the point of standstill.

It is recommended in the specialist literature in the case of the use ofblanking control, which is also termed burst-firing control, inconjunction with electric motors to compensate the fluctuations inrotational speed or torque by an additional centrifugal mass. Areference covering this application may be found in DE-Z DerElektromeister +Deutsches Elektrohandwerk, 1973, Vol. 48, Issue 18,pages 1269, 1270. According to this reference, a burst-firing controlleris understood to be a circuit arrangement in which the complete voltageand frequency are continually periodically applied to the relevantapparatus, such as the electric motor, and the apparatus is respectivelyseparated from the system between applications of complete voltage andfrequency. It goes without saying that a pulsating torque which isundesired per se is obtained in accordance with the periodic startingand stopping operations in the case of an electric motor. Thus, withelectric tools phase-gating control has been used exclusively to date inorder to achieve a torque characteristic which is as uniform as possiblein conjunction with the use of each half wave of the feeding AC voltageduring operation.

OBJECTS AND SUMMARY OF INVENTION

It is a primary object of the present invention to provide an electricscrewdriver which can be operated with a variable continuous rotationalspeed and also with a variable, jerky, or pulsing, rotational speed anda correspondingly pulsing torque.

Another object of the present invention is to provide an electricscrewdriver which counteracts the ejection forces present duringoperation of a conventional electric screwdriver.

Yet another object of the present invention is to provide an electricscrewdriver which produces a pulsed torque in the form of clocked torquepulses and rotational delays or interruptions in rotation.

The foregoing and other objects and advantages of the present inventionwill appear more clearly hereinafter.

In accordance with the present invention there is provided an electricscrewdriver in which a universal motor is driven by means of aphase-gating controller for determining variable rotational speeds. Thephase-gating controller can be switched over to wave-packet-controlledphase-gating control and the universal motor can be adjusted inoperation by the wave-packet controller to a jerky rotational speed upto the point of motor stand-still.

The phase-gating controller has a triac with an integrated circuit (IC)as the starting device and the IC is connected to an oscillator whoseoutput is connected to the enable terminal of the phase-gatingcontroller IC. The oscillator output may alternatively be connected toinfluence the trigger pulse for driving the triac.

The oscillator includes two actuators. The first actuator determines thedrive frequency of the enabling device of the phase-gating IC. Thesecond actuator determines the pulse width of the frequency. Thefrequency of the oscillator can be adjusted from 5 to 50 Hz and thepulse width of the frequency can be adjusted between 1% and 99%.

The present invention enables the counteracting of the ejection forcesduring the screwing operation and facilitates driving home buildingscrews or the like, flush in a uniform fashion. The effective screwingforces are applied by a pulsed torque. In the apparatus according to thepresent invention, clocked pulses are produced in the form of rotationaldelays as far as interruptions in rotation.

With reference to electric screwdriver operation, it is advantageous tooperate with an alternating torque which increases and decreases. Thepresent invention provides an electric screwdriver which can be operatedboth with a variable, continuous rotational speed and with a variable,jerky or pulsing rotational speed and a correspondingly pulsing torque.As a result of the variable phase-gating control, the rotational speedand torque remain adjustable in the usual way while, as a result of thevariable wave-packet-controlled phase-gating control, that is to sayowing to the wave-packet control in conjunction with the phase-gatingcontrol, it is possible for screwing forces which fluctuate in onedirection to be applied which are caused by pulses which occur in stepand take the form of delays in rotation as far as the sticking point andof the rotational speeds rising again. The centrifugal forces occurring,which in the case of screwing at constant rotational speeds, entail atendency of the screwdriver to be ejected, are counteracted.Consequently, in the case of operation with wave-packet control, theinterruptions in rotation or delays in rotation respectively mean thatthe screwdriver is placed again into the driving depressions of thescrew head with a renewed flow of power, and this produces optimumscrewing results for the user which cost less effort.

Wave-packet control can advantageously be applied in the case ofscrewing work where much force is expended, such as in the case ofscrews which are difficult to release or difficult to drive home, sinceowing to the wave-packet control, which can be adjusted variably as faras motor standstill, the maximum torque of the motor is applied to thescrewdriver when the rotational speed is re-accelerated.

BRIEF DESCRIPTION OF THE DRAWINGS

Other important objects and advantages of the present invention will beapparent from the following detailed description of the invention, takenin conjunction with accompanying drawings in which:

FIG. 1 is a schematic drawing of an electric screwdriver made inaccordance with the present invention showing a first embodiment of theinvention;

FIG. 2 is a schematic drawing similar to FIG. 1, showing a secondembodiment of the invention;

FIG. 3 is a schematic drawing similar to FIG. 1, showing a thirdembodiment of the invention;

FIG. 4 is a typical waveform produced by the wave packet controlledphase-gating control.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, wherein like reference numbers designatelike or corresponding parts throughout, there is shown in FIGS. 1-3 anelectric screwdriver, made in accordance with the present invention.

FIG. 1 shows a first embodiment of an electric screwdriver 100 made inaccordance with the present invention. Prior art electric screwdriverstypically incorporate an electronic system for controlling rotationalspeed such as a phase-gating controller. In accordance with the presentinvention, and as will be presently described, a wave-packet controlmode of operation is superimposed on the phase-gating controller and theoperation of the phase-gating controller can be switched over to awave-packet controlled phase-gating control mode.

As is shown in FIG. 1, the electric screwdriver 100 includes acommutator motor 1 which is preferably a universal motor. Connected inseries with the motor 1 is a triac 2 to which a phase-gating controller3 is connected. The phase-gating controller 3 contains a solid-stateintegrated circuit including a phase-gating integrated circuit (IC) chipas the starting and driving device for the triac 2. This phase-gatingintegrated circuit chip IC is not illustrated, but is contained withinthe rectangular block designated by the reference numeral 3. Thisstarting device has an enable terminal for the output of the gatingpulses. Connected to the phase-gating IC is an oscillator 4 whose outputis connected to the enable terminal of the phase-gating IC. Theoscillator 4 is connected to two actuators 5, 6. Actuator 5 determinesthe drive frequency of the enabling device of the IC. Actuator 6 candetermine the pulse width of the frequency. The frequency of theoscillator 4 can advantageously be adjusted from a value in the order of5 Hz to a value in the order of 50 Hz. The pulse width of the frequencymay be varied from a value in the order of 1% to a value in the order of99%.

There is a switching device 7 for determining the operating mode of thephase-gating controller 3 in the connection from the oscillator outputto the enable terminal of the phase-gating IC. The closed switchposition of the switching device 7 corresponds to thewave-packet-controlled phase-gating control mode for the motor 1. If theswitching device 7 is opened, the motor 1 is operated using thecustomary phase-gating control.

The action of the wave-packet-controlled phase-gating controller is suchthat not all half waves of the mains 102, 104 are passed at anoscillator frequency below 50 Hz. The further the frequency is below 50Hz, the fewer half waves are passed. The number of the half waves whichare sequentially passed and sequentially not passed is determined by thepulse width of the frequency.

The wave packet controlled phase-gating control enables the power to themotor to be repetitively switched on for a period of time and theswitched off for a period of time.

FIG. 4 shows a typical voltage wave form produced by the wave packetcontrolled phase-gating control. In FIG. 4 the switched "on" duration isshown by the letter E, and the switched "off" duration is shown by theletter A. The sum of the switched "on" duration E, and the switched"off" duration A is designated as the switching interval and is shown bythe letter "T".

FIG. 2 shows a second embodiment of the invention 200 including acircuit arrangement in which the wave-packet controller is constructedseparately from the phase-gating controller and is connected separatelyto the phase-gating controller.

It is usual in the case of electric screwdrivers to provide the circuitarrangement for phase-gating control completely in a mains switch 8, andso the operating element 9 of the mains switch 8 can be used to switchthe mains 102, 104 and to determine the rotational speed of the motor 1in a stepless fashion.

The wave-packet controller 10 in the second embodiment of the invention200 contains a triac. It is possible here to choose betweenwave-packet-controlled phase-gating control and the standardphase-gating control by means of a changeover element 11 or switcharranged in the connecting line of the mains 104 leading directly to themotor 1. The mode in which the wave-packet-controlled phase-gatingcontroller functions and operates is the same as was previouslydescribed with reference to FIG. 1.

FIG. 3 shows a third embodiment of the invention 300 including a circuitarrangement which circumvents the wiring of the phase-gating controller3 by virtue of the fact that here the trigger pulses for driving thetriac 2 are derived or enabled via a switching device 12. Thewave-packet controller 4 can be adjusted in a stepless fashion and/orswitched on and off via a potentiometer 13 and/or a switch 14.

In general, the phase-gating controller and/or the oscillator 4 can beimplemented in an analog manner incorporating analog components as wellas digitally in the form of a microprocessor.

The foregoing specific embodiments of the present invention as set forthin the specification herein are for illustrative purposes only. Variousdeviations and modifications can be made within the spirit and scope ofthis invention, without departing from the main theme thereof.

I claim:
 1. An electric screwdriver comprising:a universal motor;phase-gating controller means; first connection means connecting saidphase-gating controller means and said universal motor; wave-packetcontroller means; and second connection means connecting saidwave-packet controller means and said phase-gating controller means. 2.An electric screwdriver according to claim 1, in which said wave-packetcontroller means further comprises a triac.
 3. An electric screwdriveraccording to claim 1, further comprising:electrical connection meansdisposed connecting said universal motor to a source of electricalpower; switch means, disposed in said electrical connection means, withsaid switch means connected to said phase-gating controller means andconnected to said wave-packet controller means for the purpose ofchoosing between wave-packet controlled phase-gating control and purephase-gating control operation.
 4. An electric screwdriver comprising:auniversal motor; and phase-gating controller means for determiningvariable rotational speed of said universal motor, with saidphase-gating controller means capable of a wave-packet controlledphase-gating control mode and with said wave-packet controlledphase-gating control mode capable of controlling said universal motorfor a jerky rotational speed in a range of speed which extends to thepoint of standstill.
 5. An electric screwdriver according to claim 4, inwhich said phase-gating controller means comprises:a triac; integratedcircuit means with said integrated circuit means connected to said triacfor the purpose of starting said triac, with said integrated circuitmeans having an enable terminal; oscillator means, with said oscillatormeans having oscillator output means, with said oscillator output meansconnected to said enable terminal of said integrated circuit means. 6.An electric screwdriver according to claim 5, in which said triacfurther comprises gate means and with said oscillator means connected tosaid gate means of said triac.
 7. An electric screwdriver according toclaim 4, wherein said phase-gating controller means comprises analogphase-gating controller means.
 8. An electric screwdriver according toclaim 4, wherein said phase-gating controller means comprises digitalphase-gating controller means.
 9. An electric screwdriver according toclaim 4, wherein said phase-gating controller means comprises analogphase-gating controller means.
 10. An electric screwdriver comprising:auniversal motor; and phase-gating controller means for determiningvariable rotational speed of said universal motor, with saidphase-gating controller means capable of a wave-packet controlledphase-gating control mode and with said wave-packet controlledphase-gating control mode capable of controlling said universal motorfor a jerky rotational speed; with said phase-gating controller meanscomprising:a triac; integrated circuit means with said integratedcircuit means connected to said triac for the purpose of starting saidtriac, with said integrated circuit means having an enable terminal;oscillator means, with said oscillator means having oscillator outputmeans, with said oscillator output means connected to said enableterminal of said integrated circuit means; said oscillator meanscomprising;a first actuator means; and a second actuator means with saidfirst actuator means determining the drive frequency of said integratedcircuit means in said phase-gating controller means and with said secondactuator means determining the pulse width of the drive frequency ofsaid integrated circuit means in said phase-gating controller means. 11.An electric screwdriver according to claim 10, in which said oscillatormeans comprises adjustable oscillator means with said adjustableoscillator means capable of adjustment in a frequency range extendingfrom a frequency in the order of 5 Hz to a frequency in the order of 50Hz and in a pulse width range extending from a pulse width in the orderof 10% to a pulse width in the order of 80%.
 12. An electric screwdriveraccording to claim 11, further comprising:switch means for reversiblyswitching the operating mode of said phase-gating controller means, withsaid switch means connected between said oscillator output means andsaid enable terminal of said integrated circuit means in saidphase-gating controller means.
 13. An electric screwdriver according toclaim 10, in which said oscillator means comprises adjustable oscillatormeans with said adjustable oscillator means capable of adjustment in afrequency range extending from a frequency in the order of 5 Hz to afrequency in the order of 50 Hz and in a pulse width range extendingfrom a pulse width in the order of 1% to a pulse width in the order of99%.